QGIS Add Vector Layer 'layer not valid'

Question

I would like to add a vector layer to QGIS using the python console (and eventually in a separate python IDE). I have run the following code from the pyqgis cookbook:

layer = QgsVectorLayer(r'//Users//rwegener//repos//correlations//co_energy_weather_shpfiles//co_energy_weather.shp',
                   'degree_days', "memory")
if not layer.isValid():
    print('layer failed to load')

The layer loads without a problem, however the layer is not valid.

The only other pages I have found on this suggest the user to mess around with slashes so I have also tried the following ways to input the layer:

layer = QgsVectorLayer(r'/Users//rwegener//repos//correlations//co_energy_weather_shpfiles//co_energy_weather.shp',
                   'degree_days', "memory")
layer = QgsVectorLayer(r'/Users/rwegener/repos/correlations/co_energy_weather_shpfiles/co_energy_weather.shp',
                   'degree_days', "memory")
layer = QgsVectorLayer(r'\\Users\\rwegener\\repos\\correlations\\co_energy_weather_shpfiles\\co_energy_weather.shp',
                   'degree_days', "memory")

When I load the file into QGIS with the "add vector layer" button there are no problems.

I have tried also with the cookbook method:

layer = iface.addVectorLayer(r'//Users//rwegener//repos//correlations//co_energy_weather_shpfiles//co_energy_weather.shp',
                   'degree_days', "memory")

only to get the error:

AttributeError: 'NoneType' object has no attribute 'addVectorLayer'

Answer 1

Structure of QgsVectorLayer is:

layer = QgsVectorLayer(data_source, layer_name, provider_name)

So, use 'ogr' provider for shapefiles:

from qgis.core import *

# import layer
layer = QgsVectorLayer('/Users/rwegener/repos/correlations/co_energy_weather_shpfiles/co_energy_weather.shp',
                   'degree_days', 'ogr')

# test
if not layer.isValid():
    print "Layer failed to load!"
else:
    print "Layer was loaded successfully!"